KIEV, Ukraine – Rather than backing down after last week’s downing of a civilian passenger jet, Russia appears to be intervening more aggressively in the war in eastern Ukraine in what U.S. and Ukrainian officials call a dangerous escalation that will almost certainly force more robust retaliation from the United States and Europe.

Russia has increased its direct involvement in fighting between the Ukrainian military and separatist insurgents, moving more of its own troops to the border and preparing to arm the rebels with ever more potent weapons, including high-powered Tornado rocket launchers, U.S. and Ukrainian officials said Friday.

The officials, citing satellite images and other military intelligence, said Russia had positioned heavy weapons, including tanks and other combat vehicles, at several points along the border where there has been intense fighting. On Thursday, Russia unleashed artillery attacks on eastern Ukraine from Russian territory, officials in Washington and Kiev said. While Russia flatly denied accelerating its intervention on Friday, U.S. and Ukrainian officials said Moscow appeared anxious to stem gains by government forces that have succeeded in retaking some rebel-held territory.

The reported Russian moves raised the prospect of a new and more perilous chapter opening in a conflict that has inflamed the region and, with the destruction of Malaysia Airlines Flight 17 with 298 aboard, stunned the world. U.S. officials blamed a Russian-provided surface-to-air missile for the explosion and hoped that the shock of the episode would prompt the Kremlin to rethink its approach, but they are increasingly convinced that it has not.

Resolve in Europe

Obama administration officials said Russia’s rising involvement had stiffened the resolve of European leaders who have been reluctant to confront Moscow for fear of damaging their own economies. But there was no appetite for a direct military response, and it remained unclear whether the West could or would take action that may change the calculus of President Vladimir Putin of Russia as Moscow seems to devote more firepower to the fight.

U.S. and Ukrainian officials said Russia has moved beyond simply helping separatists and is now engaging directly in the war. Multiple Ukrainian military planes have been brought down in recent days by missiles fired from Russian territory, and now artillery batteries are firing from across the border into Ukraine, the officials said.

“We have detected that firing, and that does represent an escalation in this conflict,” said Josh Earnest, the White House press secretary. “It only underscores the concerns that the United States and the international community have about Russian behavior and the need for the Putin regime to change their strategy.”

U.S. officials said Russia has moved 15,000 troops near the border. Col. Steve Warren, a Pentagon spokesman, told reporters that Russia had made “imminent” plans to deliver heavier rockets to the separatists. Instead of Putin de-escalating the conflict after the Malaysia Airlines tragedy, “he’s actually taken a decision to escalate,” Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, told a security forum in Aspen, Colo.

Vice President Joe Biden called President Petro Poroshenko of Ukraine to express solidarity and pledge to coordinate with allies “about imposing further costs on Russia for its deeply destabilizing and irresponsible actions in Ukraine,” the White House said in a statement.

While the U.S. has been hesitant to make its intelligence public, Ukrainian officials have provided a daily running list of Russian incursions, including flights into Russian airspace by fighter jets and unmanned surveillance drones, as well as mortar and rocket attacks.

“We have facts of shelling of Ukrainian positions from the territory of Russian Federation,” Andriy Lysenko, a spokesman for Ukraine’s National Security and Defense Council, said at a briefing in Kiev on Friday. “We have facts on the violation of air border between Ukraine and Russia.”

Lysenko said there were active-duty Russian soldiers who had surrendered, as well as volunteer Russian fighters who had been captured. “We have information about weapons and mercenaries who have respective skills for warfare, who have been passing over from the territory of the Russian Federation,” Lysenko said.

Russia scoffs

Russia pointedly denied the U.S. allegations on Friday. In a statement, the Russian Foreign Ministry accused the U.S. of engaging in a “smear campaign.”

“All of this is accompanied by references to some ‘evidence’ allegedly available to the United States,” the ministry said. “Not one of these ‘evidences,’ however, has been shown, which is not surprising. Facts and specifics to support false allegations simply do not exist.”

On Friday, the European Union took another step toward imposing additional economic penalties focused on the financial, energy and military sectors of the Russian economy, but a letter to European leaders from Herman Van Rompuy, the president of the European Council, made clear that officials were still struggling to find a balance.

Lysenko, the military spokesman, said that Ukrainian troops were coming under increased fire from the Russian side of the border and that the Ukrainian military had recently shot down three Russian surveillance drones. One was used to target a Ukrainian base near the town of Amvrosiivka, which then quickly came under heavy rocket attack, he said.

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